Nanotube Materials for Thermal Management of Electronic Components

ABSTRACT

A heat-conducting medium for placement between a heat source and heat sink to facilitate transfer of heat from the source to the sink is provided. The heat-conducting medium can include a disk having relatively high thermal conductivity and heat spreading characteristics. The heat-conducting medium also includes a first recessed surface and an opposing second recessed surface. Extending from within each recessed surface is an array of heat conducting bristles to provide a plurality of contact points to the heat source and heat sink to aid in the transfer of heat. The recessed surfaces may be defined by a rim positioned circumferentially about the disk. The presence of the rim about each recessed surface acts to minimize the amount of pressure that may be exerted by the heat sink and the heat source against the bristles. A method for manufacturing the heat-conducting medium is also provided.

This present application is a divisional of U.S. application Ser. No.11/413,512, filed Apr. 28, 2006, which claims priority to U.S.Provisional Patent Application Ser. No. 60/684,821, filed May 26, 2005,both of which are hereby incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to systems and methods for thermalmanagement of electronic components, and more particularly to a thermaljunction device for facilitating heat transfer between a heat source anda heat sink.

BACKGROUND ART

Heat transfer for thermal management between two materials at differenttemperatures often may be accomplished by conduction, radiation and/orconvection. In the area of electronics, in a narrow region at, forinstance, an interface between a die lid (e.g., commonly acopper-tungsten material) of the integrated circuit and the heat sink,the temperature present in the integrated circuit (IC) can typically bebetween about 40° C. to 150° C. For such a situation, thermal managementmay typically be accomplished through conduction. However, the use offlat plates at the interface to facilitate the heat transfer from theintegrated circuit to the heat sink has not been optimal. In particular,the use of a flat plate may provide only between 20 to 50 points ofcontact to the integrated circuit and/or the heat sink. As a result, theheat that flows out of the hot integrated circuit can only pass throughthese few contact spots.

To enhance the transfer of heat to the heat sink, current technologyusually involves placing a thermally conducting grease between the dielid of an integrated circuit and the heat sink device. The heat sinkdevice, in general, may be of any type, including a passive heat sink, aPeltier cooler, a refrigerated copper block, a heat pipe, or an activefan type, or a copper block in which embedded heat pipes can carry heatto a water-cooled bus outside of the system.

Presently, thermal greases that are commercially available typicallycontain silver powder or silver flake, and may be used by applying tomachined, and occasionally, lapped heat sinks and integrated circuitlids. However, the thermal conductivity of these commercially availablegreases at best may only be about 9 watts/m-deg K. For example, (i)Arctic Silver III has a thermal conductivity of >9.0 W/m-deg K, (ii) AOSThermal Compounds has a thermal conductivity of about 7.21 W/m-deg K,(iii) Shin-Etsu G751 has a thermal conductivity of about 4.5 W/m-deg K,(iv) AOS Thermal Compounds HTC-60 has a thermal conductivity of about2.51 W/m-deg K, (v) Thermagon T-grease has a thermal conductivity ofabout 1.3 W/m-deg K, and (vi) Radio Shack Thermal Grease has a thermalconductivity of about 0.735 W/m-deg K. As illustrated in FIG. 1, thereexists, generally, a 20 degrees difference between the heat source andthe heat sink. Such a difference may indicate a thermal resistance atthe junction and suggests that the potential to carry heat to the sinkmay be hurt by the poor interface provided by the grease.

It has been known that metal fiber structures and material can provide alow loss connection at greatly reduced forces, thereby providinghigh-efficiency, low force electrical contact. Based on simple laws ofphysics, the capability of fiber brushes to efficiently transferelectrical current across interfaces, which can be in relative motion orat rest, is paralleled by their capability to similarly transfer heat.In particular, since they operate at low loads and have very lowresistance, they can dissipate relatively much less heat. Moreover, thefiber brushes can provide a substantial amount contact points betweenthe heat source and heat sink to permit efficient heat transfer. As aresult, metal fiber brushes have been used in a thermal interface asheat conduits for cooling or heating purposes. (U.S. Pat. No. 6,245,440)

Recently, carbon nanotubes have been used in thermal management. It hasbeen shown that the thermal conductivity of carbon nanotubes is over2980 watts/m-deg K as compared to thermal grease, which is only about 9watts/m-deg K maximum (Thermal Conductivity of Carbon Nanotubes byJianwei Che*, Tahir Cagin, and William A. Goddard III Materials andProcess Simulation Center California Institute of Technology Pasadena,Calif. 91106E-mail: jiche@caltech.edu. Even higher numbers are reportedby Tomanek (VOLUME 84, NUMBER 20 PHYSICAL REVIEW LETTERS 15 MAY 2000“Unusually High Thermal Conductivity of Carbon Nanotubes,” Savas Berber,Young-Kyun Kwon,* and David Tomanek).

In addition, U.S. Pat. No. 6,891,724, discloses the use of carbonnanotubes deposited on a CVD diamond coated thermally heat die. Inparticular, a CVD diamond coating is placed on a heat die, and the diesubsequently coated with carbon nanotubes.

In Carbon nanotube composites for thermal management, M. J. Biercuk, M.C. Llaguno, M. Radosavljevic, J. K. Hyun, and A. T. Johnson, Departmentof Physics and Astronomy and Laboratory for Research on the Structure ofMatter, University of Pennsylvania, Philadelphia, Pa. 19104—AppliedPhysics Letters—Apr. 15, 2002—Volume 80, Issue 15, pp. 2767-2769, theauthors discussed adding a small amount of carbon nanotubes, withoutsurface modification, to an epoxy matrix to improve heat transfer. InStudy of Carbon Nanofiber Dispersion for Application of Advanced ThermalInterface Materials, Xinhe Tang*, Ernst Hammel, Markus Trampert, KlausMauthner, Theodor Schmitt, Electrovac GmbH, Aufeldgasse 37-39, 3400Klosterneuburg, Austria and Jurgen Schulz-Harder, Michael Haberkorn,Andereas Meyer, Curamik Electronics GmbH, Am Stadtwald 2, 92676Eschenbach, Germany, the authors described how adding carbon nanotubesto thermal grease improves thermal performance.

Others have developed approaches to aligning nanotubes in arrays forother applications. For example, Jung, Y. J., et al. “Aligned CarbonNanotube-Polymer Hybrid Architectures for Diverse Flexible ElectronicApplications.” Nano Lett., 6 (3), 413-418, 2006, discloses a nanotubefilled polymer but does not include thermal applications.

SUMMARY OF THE INVENTION

The present invention, in one embodiment, is directed to aheat-conducting medium for placement between a heat source and heat sinkto facilitate transfer of heat from the source to the sink.

In one embodiment, the heat-conducting medium includes a disk, made froma material having a relatively high thermal conductivity characteristic,for placement between a heat source and a heat sink. The disk may alsohave a heat spreading characteristic. The heat-conducting medium furtherincludes a first recessed surface on the disk for placement adjacent theheat source and an opposing second recessed surface on the disk forplacement adjacent the heat sink. The heat-conducting medium may furtherinclude an array of heat conducting bristles extending from within thefirst and second recessed surfaces. In an embodiment, the recessedsurfaces may be defined by a rim positioned circumferentially about thedisk. The presence of the rim about each recessed surface acts toprovide a spacer between the heat source and heat sink and to minimizethe amount of pressure that may be exerted by the heat sink and the heatsource against the bristles. The bristles, in an embodiment, may extendbeyond the rim on the respective surface from which the bristle arepositioned to provide a plurality of contact points to the heat sourceand to the heat sink to aid in the transfer of heat.

The present invention, in another embodiment, is directed to asubstantially flexible heat-conducting medium. This heat-conductingmedium, in one embodiment, includes a flexible member made from an arrayof interweaving carbon nanotubes. The flexible member may include anupper surface against which a heat source may be placed, an opposinglower surface, and edges about the member designed for coupling to aheat sink toward which heat from the heat source can be directed. Theheat-conducting medium also includes a pad for placement on the uppersurface of the member to provide structural support to the member. In anembodiment, a second pad may be provided against the lower surface ofthe member to provide additional support to the flexible member. Theheat-conducting medium may further include a heat spreader positionedadjacent the heat source and the upper surface of the member tofacilitate radial transfer of heat from the heat source to a wider areaon the member. To the extent desired, a second heat spreader may beprovided against the lower surface of the flexible member to enhancespreading of heat from the heat source radially along the flexiblemember.

In accordance with another embodiment, the present invention provides amethod for manufacturing a heat-conducting medium. In one embodiment, adisk having opposing recessed surfaces and a relatively high thermalconductivity characteristic may initially be provided. Next, a pluralityof catalyst particles may be deposited into the recessed surfaces. In anembodiment, prior to depositing the catalyst particles, the recessedsurfaces may be coated with a material that can enhance attachment ofthe particles to the recessed surfaces. Thereafter, the catalystparticles may be exposed to a gaseous carbon source, and from the uptakeof carbon by the catalyst particles, may be allowed to permit growth ofnanotubes from the recessed surfaces. Once the nanotubes have extendedbeyond the recessed surfaces, the growth of the nanotubes may beterminated.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a table with examples of commercial conductinggreases and their thermal conductivity.

FIG. 2 illustrates a cross-sectional perspective view of a heatconducting medium in accordance with one embodiment of the presentinvention.

FIG. 3 illustrates a cross-sectional view of the heat-conducting mediumin FIG. 2 having an array of nanotubes positioned within opposingrecesses.

FIG. 4 illustrates a cross-sectional view of a heat-conducting medium inaccordance with another embodiment of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides, in one embodiment, a medium for thermalmanagement of electronic components. The medium, in an embodiment may bea thinly designed device that may be place at a thermal junction betweena heat source, such as an integrated circuit, and a heat sink tofacilitate heat transfer from the heat source to the heat sink

With reference now to FIG. 2, the present invention provides, in oneembodiment, a heat-conducting medium 20 for carrying thermal energy awayfrom a heat source. The heat-conducting medium 20, in an embodiment,includes a substantially thin disk 21 designed so that it may be placedin a narrow region at, for instance, an interface between a lid of aheat generating integrated circuit (IC) and a heat sink. To that end,disk 21 may be provided with a thickness ranging from about 2 millimeter(mm) to about 4 mm. Of course the thickness of the disk 21 may varyaccording to the particular application and placement. In addition, disk21 may be made from a material having relatively high thermalconductivity and heat spreading characteristics, so as to facilitateheat transfer from the heat generating IC to the heat sink. Examples ofsuch a material include copper, aluminum, beryllium, or a combinationthereof. In one embodiment of the invention, disk 21 may be made fromsubstantially high purity copper. Of course other materials may be used,so long as they provide disk 21 with high thermal conductivity and heatspreading characteristics.

As illustrated in FIG. 2, disk 21 of heat-conducting medium 20 mayinclude a first surface 211 for placement adjacent a heat source. Disk21 may also include an opposing second surface 212 for placementadjacent a heat sink. In that manner, the first and second surfaces 211and 212 may act as a conduit to pull heat from a heat source to the heatsink. First surface 211, in an embodiment, may be designed to include arecessed surface 23 defined by rim 25, while the second surface 212 maybe designed to include a recessed surface 24 defined by rim 26. Recessedsurfaces 23 and 24 may be situated, in an embodiment, approximately inthe center of disk 21 for accommodating an array of carbon nanotubebristles 30 (see FIG. 3). To that end, the recessed surfaces 23 and 24may be provided with a depth that is measurably less than the length ofthe nanotube bristles 30. In one embodiment, the depth of each recessedsurface may be approximately between 100 microns and 500 microns ormore, depending of the particular application and location at which thedisk may be placed.

Rims 25 and 26, situated circumferentially about disk 21, may beprovided, in an embodiment, to act as a spacer between the heat sink andthe heat source. The presence of rims 25 and 26 on disk 21 may also actto limit the amount of pressure or provide the appropriate amount ofpressure that may be exerted by the heat sink and heat source againstthe nanotube bristles 30. To the extent that a significant amount ofpressure is exerted on the nanotube bristles 30, that is, significantlymore than necessary, the bristles 30 may be damaged and the transfer ofheat may be compromised.

It should be appreciated that the recessed surfaces 23 and 24 may becreated by machining, coined on a coin press, or any other methods knownin the art. In addition, although illustrated as circular in shape, thedisk 21 may be provided with any geometric shape, for instance, square,hexagonal, octagonal etc., so long as the disk can act as an interfacebetween a heat source and a heat sink.

Looking now at FIG. 3, the heat-conducting medium 20 may also include anarray of heat-conducting bristles 30 situated within recessed surfaces23 and 24. The presence of the array of bristles 30, which may beflexible in nature, can overcome the low number of contact spots betweenthe heat source and heat sink typically observed in prior art flatplate. In accordance with one embodiment of the present invention, theflexible bristles 30 may be situated substantially transverse to therecessed surfaces 23 and 24, and may extend or protrude from withinrecessed surfaces 23 and 24 to about 10 microns to about 100 microns orslight more beyond rims 25 and 26 of disk 21. In this way, the tips ofthe bristles 30 can maintain substantially good contact with the heatsource and heat sink during use.

Moreover, because good thermal conductivity is necessary, bristles 30,in an embodiment, may be made from carbon nanotubes. The carbonnanotubes for use in connection with the heat-conducting medium 20 ofthe present invention may be single wall nanotubes or multi-wallnanotubes, and may, in an embodiment, be less than approximately 50 nmin diameter. It should be noted that by allowing the bristles 30 toextend beyond rims 25 and 26, disk 21, when situated within the narrowregion or junction between the heat source and heat sink, can permit theheat sink and the heat source (e.g., lid of the IC) to both bear againstrims 25 and 26 on disk 21, thereby bending the protruding flexiblenanotube bristles 30 in such manner so as to maintain good thermalcoupling to both the heat source and the heat sink.

By employing an array of nanotube bristles 30, the number of contactpoints can be significantly increased. In one embodiment, the number ofcontact points provided may range on the order of up to about 10⁸ persquare centimeter or higher. Moreover, if, for instance, only about 20percent of the surface of the apparent contact area is filled withnanotube bristles 30, then an approximate thermal conductivity can beestimated to be about 0.20*2980 watts/m-deg. K or about 600 watts/m-deg.K, which compares rather well with currently available 9 watts/m-deg Kfor thermally conducting grease.

It should be appreciated that although the amount of bristles 30illustrated in FIG. 3 may be substantially similar on recessed surface23 and recessed surface 24, the medium 20 can be designed so that theamount of bristles 30 on each surface may be uneven relative to oneanother. For example, if the heat source is a small die or smallintegrated circuit, the heat source side (i.e., surface 23) of disk 21can be relatively smaller with fewer bristles 30 in comparison to theheat sink side (i.e., surface 24) of disk 21. With such a design theheat-conducting medium 20 may also act as a heat spreader, spreadingheat from the smaller heat source surface 23 radially along the medium20 to the larger heat sink surface 24. In addition, to the extent thatthere may be fewer bristles 30 on recessed surface 23, recessed surface23, which may generally be similar in size to recessed surface 24, maybe made to be smaller relatively to recessed surface 24. To provide arelatively smaller recessed surface 23, rim 25 may, in an embodiment, bemade to be radially thicker.

The array of bristles 30, in an embodiment, may be provided on opposingrecessed surfaces 23 and 24 of the disk 21 by various means known in theart. In one approach, coatings may be placed on the heat-conductingmedium 20 in the region where the nanotube bristles 12 may grow (i.e.,the recessed surfaces 23 and 24). These coatings may be selected so asnot to react with the material from which the heat-conducting medium 20may be made. The coatings may include, for example, iron, molybdenum,alumina, silicon carbon, aluminum nitride, tungsten or a combinationthereof. In one embodiment, the coatings can be applied onto therecessed surfaces 23 and 24 by any means known in the art, so that adense substantially pore-free deposit may be produced. In addition,certain catalysts may be deposited onto the coatings. Deposition of thecatalysts onto the coatings can be accomplished, in an embodiment, byspraying, painting, screen-printing, evaporation or by any process knownin the art. Catalysts that may be used in connection with theheat-conducting medium 20 of the present invention may generally bemagnetic transition metals, examples of which include as iron, cobalt,nickel or a combination thereof. The catalyst particles may subsequentlybe exposed to a gaseous carbon source, such as that associated with achemical vapor deposition (CVD) process, a well-known process in theart, and allowed to take up carbon to permit growth of nanotubestherefrom.

The heat conducting medium 20 of the present invention can overcome anumber of problems, including a low number of contact spots observed inprior art flat plates by employing an array of flexible nanotubebristles 30. In particular, when placed within a junction between theheat source and heat sink, the bristles 30 on disk 21 may be pressedonto a hot surface of the heat source and act to carry heat away or actas a heat spreader from the surface of the heat source to a cooler heatsink in a manner that results in a low thermal resistance path betweenthe heat source and the heat sink. In particular, heat can travel alongthe nanotube bristles 30 and across the thin disk 21 to the contactingsurfaces with substantially low contact resistance. Presently thermalresistance between such heat source and a heat sink can be as high as 20degrees Centigrade. It is believed that this thermal resistance can bereduced to a small fraction of this amount using the present invention.The consequences can be that the power dissipated can be increased, andthe temperature of the heat source can also be reduced.

In addition, by employing an array of nanotube bristles 30, thetemperature gradient required to drive heat to the heat sink can bereduced to much less than 20° C. Furthermore, rough interfaces may beaccommodated so that lapping the interfaces may not be required. Inother words, grinding of the rough interfaces may be minimized.Moreover, differences in the coefficient of thermal expansion betweenthe heat source (e.g., lid of the IC) and the heat sink may beaccommodated, so that, for example, expensive copper tungsten heatspreaders and the required brazing process can be eliminated. Theheat-conducting medium 20 with an array of nanotube bristles 30 can alsobe used as a drop-in substitute for “conducting grease” taking up only afew mm in vertical geometry.

Looking now at FIG. 4, there is illustrated another heat-conductingmedium 40 for thermal management in accordance with further embodimentof the present invention. The heat-conducting medium 40, in anembodiment, includes a flexible member 41, such as a mat or textilematerial made from carbon nanotubes. In other words, carbon nanotubesmay be wound into fibers or yarns and the fibers or yarns formed orwoven into a mat or textile material 41. The heat-conducting medium 40,in one embodiment, may be infiltrated with polyamide 42, epoxy, otherpolymers or a combination thereof.

The heat-conducting medium 40 may also include a pad 43 placed on uppersurface 44 of textile material 41 to support a heat source, such as IC46. The presence of pad 43 may also provide structural support to theflexible member 41. To the extent desired, pad 43 may also be placedagainst lower surface 45 of textile material 41 to provide additionalstructural support to the flexible member 41.

As illustrated in FIG. 4, the heat-conducting medium 40 may be used as aheat conducting medium in the manner similar to that discussed withmedium 20 above. In particular, a heat source, such as IC 46 may beplaced onto heat conducting medium 40 against the upper surface 44 ofthe flexible member 41. To that end, heat generated from the heat sourcemay be carried by the flexible member 41 toward its edges 411 designedto couple to a heat sink, such as water cooling pipe 47, a heat pipe, orany material that passively conducts heat along the flexible member 41away from the heat source 46.

In another embodiment, the heat-conducting medium 40 may further includea heat spreader 48 placed adjacent to the heat source 46 and the uppersurface 44 of the flexible member 41. Heat spreader 48, in oneembodiment, may be situated between the heat source 46 and the uppersurface 44 of the textile material 41. As such, heat spreader 48 may actto facilitate the radial transfer of heat from the heat source 46quickly to a wider area on the textile material 41 than otherwise maybe, so that the heat from the heat source 46 may subsequently be carriedto heat sink 47. As shown in FIG. 4, an additional heat spreader 49 maybe positioned against the lower surface 45 of the textile material 41 tofurther facilitate the spreading of heat from the heat source 46radially along the textile material 41. In an embodiment, the additionalheat spreader 49 may be placed directly below the heat spreader 48 onthe upper surface of the flexible member 41.

To the extent desired, the textile material 41 may also, in oneembodiment of the present invention, be incorporated within, forexample, a printed circuit board for diverting heat from a heat source.Alternatively, the textile material 51 may not be a textile ortextile-like in nature, but rather, be part of a thermally conductivecomposite, such as a highly loaded carbon-carbon composite, where thefiber loading may be above about 50%, and further be directional in thedirection of the heat flux.

While the invention has been described in connection with the specificembodiments thereof, it will be understood that it is capable of furthermodification. Furthermore, this application is intended to cover anyvariations, uses, or adaptations of the invention, including suchdepartures from the present disclosure as come within known or customarypractice in the art to which the invention pertains.

1. A heat-conducting medium for thermal management, the mediumcomprising: a disk for placement between a heat source and a heat sink;a first recessed surface on the disk for placement adjacent the heatsource; an opposing second recessed surface on the disk for placementadjacent the heat sink; and an array of heat conducting bristlesextending from within the first and second recessed surfaces, such thatthe bristles in the first recessed surface provides a plurality ofcontact points to the heat source and the bristles in the secondrecessed surface provides a plurality of contact points to the heatsink.
 2. A medium as set forth in claim 1, wherein the disk is made froma material having a relatively high thermal conductivity characteristic.3. A medium as set forth in claim 1, wherein the disk is made from amaterial having a heat spreading characteristic.
 4. A medium as setforth in claim 1, wherein the disk is made from one of copper, aluminum,beryllium, or a combination thereof.
 5. A medium as set forth in claim1, wherein each of the first and second recessed surfaces is defined bya rim positioned circumferentially about the disk.
 6. A medium as setforth in claim 5, wherein the rim acts as a spacer between the heat sinkand the heat source.
 7. A medium as set forth in claim 5, wherein therim acts to an amount of pressure that may be exerted by the heat sinkand the heat source against the array of bristles.
 8. A medium as setforth in claim 1, wherein each of the first and second recessed surfacesincludes a depth that is measurably less than the length of the array ofbristles extending therefrom.
 9. A medium as set forth in claim 1,wherein each of the first and second recessed surfaces includes a depthbetween approximately 100 microns and approximately 500 microns.
 10. Amedium as set forth in claim 1, wherein the first and second recessedsurfaces are substantially similar in size.
 11. A medium as set forth inclaim 1, wherein the first and second recessed surfaces are different insize.
 12. A medium as set forth in claim 11, wherein the first recessedsurface is smaller in size than the second recessed surface to permitheat from a small heat source to be spread to a relatively larger heatsink.
 13. A medium as set forth in claim 1, wherein each array ofbristles is situated substantially transverse to the respective recessedsurface from which it extends.
 14. A medium as set forth in claim 1,wherein each array of bristles extends about 10 microns to about 100microns beyond its respective recessed surface.
 15. A medium as setforth in claim 1, wherein the number of contact points provided by eacharray of bristles ranges on the order of up to about 10⁸ per squarecentimeter or more.
 16. A medium as set forth in claim 1, wherein thearrays of bristles extending from the first and second recessed surfacesare substantially similar in number.
 17. A medium as set forth in claim1, wherein the arrays of bristles extending from the first and secondrecessed surfaces are different in number.
 18. A medium as set forth inclaim 1, wherein the bristles extending from the first recessed surfaceis less in number than the bristles extending from the second recessedsurface to permit heat from a small heat source to be spread to arelatively larger heat sink.
 19. A medium as set forth in claim 1,wherein the bristles are made from carbon nanotubes.
 20. Aheat-conducting medium for thermal management, the medium comprising: adisk having a first side for placement adjacent a heat source and anopposing second side for placement adjacent a heat sink; a rimpositioned circumferentially about each side of the disk; a firstrecessed surface defined by the rim on the first side of the disk; asecond recessed surface defined by the rim on the second side of thedisk; and an array of heat conducting bristles extending from within thefirst and second recessed surfaces, such that the bristles in the firstrecessed surface provides a plurality of contact points to the heatsource and the bristles in the second recessed surface provides aplurality of contact points to the heat sink.
 21. A medium as set forthin claim 20, wherein the disk is made from a material having arelatively high thermal conductivity characteristic.
 22. A medium as setforth in claim 20, wherein the disk is made from a material having aheat spreading characteristic.
 23. A medium as set forth in claim 20,wherein the rim acts to an amount of pressure that may be exerted by theheat sink and the heat source against the array of bristles.
 24. Amedium as set forth in claim 20, wherein the first and second recessedsurfaces are of similar size defined by their respective rim.
 25. Amedium as set forth in claim 20, wherein the first and second recessedsurfaces are different in size defined respectively by different sizedrims.
 26. A medium as set forth in claim 20, wherein each array ofbristles extends slightly beyond the rim on the respective surface, suchthat the medium can accommodate differences in coefficient of thermalexpansion between the heat source and the heat sink.
 27. A medium as setforth in claim 20, wherein the arrays of bristles permit the medium toaccommodate rough interfaces between the heat source and heat sink, sothat lapping the interfaces can be minimized.
 28. A medium as set forthin claim 20, wherein the arrays of bristles extending from the first andsecond recessed surfaces are substantially similar in number.
 29. Amedium as set forth in claim 20, wherein the arrays of bristlesextending from the first and second recessed surfaces are different innumber.
 30. A method for manufacturing a heat-conducting medium forthermal management, the method comprising: providing a disk havingopposing recessed surfaces and a relatively high thermal conductivitycharacteristic; depositing a plurality of catalyst particles into therecessed surfaces; exposing the catalyst particles in the recessedsurfaces to a gaseous carbon source; allowing uptake of carbon by thecatalyst particles to permit growth of nanotubes from the recessedsurface; and terminating the growth of the nanotubes when they extendbeyond the recessed surfaces.
 31. A method as set forth in claim 30,wherein prior to the depositing the catalyst particles, the methodincludes coating the recessed surfaces with a material that enhancesattachment of the particles to the recessed surfaces.
 32. A method asset forth in claim 31, wherein, in the step coating, the materialincludes one of iron, molybdenum, alumina, silicon carbon, aluminumnitride, tungsten, or a combination thereof.
 33. A method as set forthin claim 30, wherein, in the step of depositing, the catalyst particlesare made from magnetic transition metals.
 34. A method as set forth inclaim 30, wherein, in the step of depositing, the catalyst particlesinclude one of iron, cobalt, nickel, or a combination thereof.
 35. Amethod as set forth in claim 30, wherein the step of exposing includesthe use of chemical vapor deposition for growing the fibers.